1. Field of the Invention
The present invention relates to an improvement of a passage wall cooling structure in an outboard engine system including a case member which is disposed below an engine body and integrally provided with an oil pan opening upwards, and a passage wall extending vertically to define an exhaust gas passage through which an exhaust gas flows, so that the passage wall is cooled to prevent the temperature of the oil pan from being raised by the exhaust gas flowing through the exhaust gas passage.
2. Description of the Related Art
Such a passage wall cooling structure in an outboard engine system is conventionally known from Japanese Utility Model Publication No. 54-2556 and Japanese Patent Application Laid-open No. 58-183384 and the like, for example.
In the system disclosed in the above Japanese Utility Model Publication No. 54-2556, however, a water channel is defined by a recess provided in the outer wall of the case member and a side lid incorporated in the recess, so that a portion of cooling water in the water channel is incorporated into an exhaust gas in the exhaust gas passage. In order to drop the temperature of the passage wall, the amount of cooling water incorporated into the exhaust gas must be increased, and a back flow of a splash of the cooling water due to the pulsation of the exhaust gas must be taken into consideration depending on a position of incorporation of the cooling water, resulting in a troublesome handling.
In the system disclosed in the above Japanese Patent Application Laid-open No. 58-183384, an inner tube portion which is the passage wall defining the exhaust gas passage and an intermediate tube portion surrounding the inner tube portion, are disposed within an outer tube portion forming an outer wall of the oil pan, and the cooling water passage is defined between the inner tube portion and the intermediate tube portion. However, such a structure is produced by molding, but to provide a draft, the area of an opening in one end of each of the outer, inner and intermediate tube portions must be larger than that in the other end. For this reason, a wasteful wall is left to bring about an increase in weight and an increase in size of the case member.
The present invention has been accomplished with such circumstances in view, and it is an object of the present invention to provide a passage wall cooling structure in an outboard engine system, wherein the passage wall integral with the oil pan can be cooled effectively, while avoiding the increase in size and the increase in weight.
To achieve the above object, according to the present invention, there is provided a passage wall cooling structure in an outboard engine system comprising a case member which is disposed below an engine body and integrally provided with an oil pan opening upwards, and a passage wall extending vertically to define an exhaust gas passage through which an exhaust gas flows, wherein a portion of an outer wall of said case member is formed by the passage wall, the outer wall of the case member being provided with a recess which is covered with a plate member fixed to the outer wall of the case member, with a portion of said passage wall facing said recess, and a cooling water passage, through which cooling water flows, is defined between the plate member and the recess.
With such arrangement, the passage wall can be cooled effectively by the cooling water flowing through the cooling water passage to prevent the rise in temperature of the oil pan. Moreover, a portion of the outer wall of the case member is formed by the passage wall, and the recess is provided in the outer wall of the case member with a portion of the passage wall facing the recess. Therefore, it is unnecessary to take account of a draft provided in the molding for forming the cooling water passage, thereby avoiding increases in size and weight of the case member.
The above and other objects, features and advantages of the invention will become apparent from the following description of the preferred embodiment taken in conjunction with the accompanying drawings.